The present invention relates to the field of glass/ceramic composites and more particularly to glasses based on Al2O3xe2x80x94CaO which include TiB2 phase.
It has long been found that alumininate glasses based on Al2O3 and CaO transmit infra-red light. Although there are other glasses which do this, (e.g., tellurites, germanates, etc.), the aluminates have a better combination of properties (optical, mechanical and thermal) as well as lower cost which makes them the attractive candidates when transmission out to about 6 xcexc is required. Glasses based on CaOxe2x80x94Al2O3 may be good candidates for this type of application since they are infrared light transmittable, have high softening temperature ( greater than 1000xc2x0 C.) and good mechanical and thermal properties, and have lower costs compared to other similar materials Unfortunately, the CaOxe2x80x94Al2O3 system is readily devitrified and requires high critical cooling rate in order to form stable glass.
To promote glass formation, silica (SiO2) has been added with some success. For instance, a composition of 48.6Al2O3-44.8CaO-6.6 SiO2 (wt. %) was found to be devitrification free [see, J. E. Stanworth,, J. Soc. Glass Technol., 1948, vol. 32, pp. 154-172]. Glasses based on Al2O3-CaO without silica but with a complex composition were also produced [see, K. H. Sun, Glass Ind., 1949, vol. 30(4), pp.199-200, 232.]. Later on, silica free glasses with much simpler composition [e.g., 47Al2O3-43CaO-10BaO (wt. %)] were reported to be formed by Florence et al. [see, J. M., Florence, et al., J. Res. Natn. Bur. Stand., 1955, vol. 55, pp.231-237]. Finally, many glass compositions based on Al2O3-CaO and containing Na2O, K2O, MgO, BaO, La2O3 and Fe2O3 were produced by Hafner et al. [H. C. Hafner et al., J. Am Ceramic Soc., vol 41(8), 1958, pp.315-323]. However, these glasses were also devitrified to a different degree and pure glasses were obtained only after up to 5 mol % SiO2 was added.
This invention uses a novel technique to produce glasses based on Al2O3-CaO with the possibility of lower costs than the traditional processing technique. The new glasses created in this invention have a simpler composition than those produced by Hafner et al. mentioned above. The glass matrix is based on Al2O3-CaO-BaO-SiO2 which contains higher A2O3 content and exhibits high softening temperature and better corrosion resistance. It also contains crystalline TiB2 phase which appears as precipitates. The material can also be produced to contain relatively high porosity which finds use in applications requiring light weight and high temperature corrosion resistance.
The traditional technique of manufacturing Al2O3xe2x80x94CaO glasses involves fusion of the oxides at 1400-1500xc2x0 C. for a long time followed by casting and shaping to desired shapes. On the other hand, this invention uses the Self-propagating High Temperature Synthesis (SHS) or Combustion Synthesis technique which is a favorable technique to produce glassy materials since the technique offers instant high combustion temperature without a furnace.
The application of the SHS technique to produce various crystalline materials has been demonstrated in many published articles and in a number of review articles [e.g., Moore and Feng, Progress in Materials Science, Vol. 39, 243-316 (1995); Munir and Anselmi-Tamburini, Materials Science Reports, vol.3, 277-365 (1989), Yi and Moore, J. Mater. Sci., vol. 25, 1159-1168 (1990)]. Simply speaking, the technique uses reactant powders to form a green pellet which is then ignited by an external heat source to generate chemical reactions, producing the end product in-situ. The SHS process can be realized by two modes, i.e., propagation (or combustion) mode and simultaneous (or thermal explosion) combustion mode. In the propagation mode, the reactants are ignited by an external heat source. Once ignited, the highly exothermic reaction ignites the next adjacent reactant layer by itself thereby generating a self-sustaining wave propagating toward the un-reacted part. In the simultaneous combustion mode, all the reactants are heated uniformly until the combustion reaction is initiated simultaneously throughout the whole pellet. A combustion synthesis reaction is defined by mainly three parameters: ignition temperature, which is the temperature at which the reaction rate becomes appreciable and self-sustaining; combustion temperature, which is the maximum temperature achieved; and combustion wave velocity, which is the overall combustion rate. However, the state of green reactants, (i.e. particle size, green density, reaction environment etc.) has a profound influence on the combustion synthesis process.
The present authors synthesized series of glass ceramic composites based on Al2O3xe2x80x94B2O3xe2x80x94BaO and Al2O3xe2x80x94B2O3xe2x80x94MgO glasses using the SHS technique (see Yi et al, U.S. Pat. No. 5,792,417 and Yi et al., U.S. Pat. application Ser. No. 09/351,227). The present invention is a further continuation in this work which reveals processing of another series of glass-ceramic composites based on CaOxe2x80x94Al2O3xe2x80x94Xxe2x80x94Y. In these series X and Y can be any metal or any metal oxide. For purposes of this application, metal is defined to include Si. However in this document only examples of X and Y being SiO2 and BaO are given.
Development of glass (Al2O3xe2x80x94CaOxe2x80x94Xxe2x80x94Y) ceramic (TiB2) composites and methods of synthesizing them represents a great improvement in the field of glasses and ceramics and satisfies a long felt need of the ceramic engineer.
This invention is a method for synthesising glass-ceramic composites, comprising the following steps:
1. mixing the reactant powders in proportion according to one of the following reactions:
3TiO2+3B2O3+10Al+xcex1CaOxe2x86x923TiB2+5Al2O3+xcex1CaO+Qxe2x80x83xe2x80x83(1) 
3TiO2+3B2O3+10Al+xcex1CaO+xcex2Xxe2x86x923TiB2+5Al2O3+xcex1CaO+xcex2X+Qxe2x80x83xe2x80x83(2) 
3TiO2+3B2O310Al+xcex1CaO+xcex2X+xcex3Yxe2x86x923TiB25Al2O3+xcex1CaO+xcex2X+xcex3Y+Qxe2x80x83xe2x80x83(3) 
TiH2+B2O3+2Al+xcex1CaO+xcex2X+xcex3Yxe2x86x92TiB2+Al2O3+xcex1CaO+xcex2X+xcex3Y+H2(g)+Qxe2x80x83xe2x80x83(4) 
Ti+B2O3+2Al+xcex1CaO+xcex2X+xcex3Yxe2x86x92TiB2+Al2O3xcex1CaO+xcex2X+xcex3Y+Qxe2x80x83xe2x80x83(5) 
where X and Y represent any metal oxide or any metal. For the purposes of this application metal is defined to include Si. It is desirable to limit the amount of silica to be added since it reduces the infrared transmission and heavier oxide such as BaO is preferred.
2. pressing the mixed powders into pellets; and
3. igniting the pellets in an argon atmosphere by resistant heating of a W-wire.
The glasses or partially devitrified glasses produced are based on the A2O3xe2x80x94CaO system and may contain one or more other substances resulting in A2O3xe2x80x94CaOxe2x80x94Xxe2x80x94Y. The preferred composition of the products is Al2O3xe2x80x94CaOxe2x80x94BaOxe2x80x94SiO2. The products also contain a ceramic phase, the ceramic phase being TiB2. The preferred product has the following composition:
TiB2: 13-26 wt. %
Al2O3: 29-50 wt. %
CaO: 16-42 wt. %
SiO2: 0-35 wt. %
BaO: 0-17 wt. %.
An appreciation of the other aims and objectives of the present invention and an understanding of it may be achieved by referring to the accompanying drawings and description of a preferred embodiment.